Total internal reflection

Question 1

What is the critical angle?

Answer 1

the angle at which the light ray refract along the boundary

Question 2

What happens if the angle of incidence is increased past the criticial angle?

Answer 2

total internal relfection will occur

Question 3

What other condition is needed for total internal reflection to occur?

Answer 3

light must meet medium with a lower refractive index - optically less dense

Question 4

What is the angle of refraction at the critical angle (i_c)? What does this tell us?

Answer 4

90 because the ligh ray emergres along the boundary

therefore n₁ sin i_c = n₂ sin90

where n1 is the refractive index of the incident substance

since sin90 = 1 then

sinθ_c = n₂ / n₁

Question 5

Why do diamonds sparkle when white light is directed at them?

Answer 5

high refractive index - greatly separates the colours

critical angle of 24°, so the light may be totally internally reflected many times before it emerges

Question 6

How does light travel through an optical fibre?

Answer 6

totally internally reflected each time it reaches the fibre boundary, even where the fibre bends, unless the radius of the bend is too small

Question 7

What is the structure for optical fibres? outline purpose

Answer 7

• the fibres need to be transparent to prevent absorbtion
• core
  
  • Core is transmission medium for em waves to progress (by total internal reflection)
• cladding
  
  • Cladding provides lower refractive index so that total internal reflection takes place
  • And offers protection of boundary from scratching which could lead to light leaving the core

Question 8

when does pulse dispersion occur?

Answer 8

• if white light is used instead of monochromatic light
• • violet light travels more slowly than red light in glass
• • the difference in speed would cause pulses to become longer

Question 9

How does a medical endoscope work?

Answer 9

contains two bundles of fibres

inserted into body cavity

one is illyminiated

one contains a lens to form an image of inside the body on the end of the fibre

has to be a coherent bundle - fibre ends are in the same relative position

Question 10

WHy is it described a TOTAL internal reflection

Answer 10

some partial reflection occurs with the angle of incidence = angle of reflection

Question 11

What is the advantage of using a smaller diameter core in an optical fibre

Answer 11

• less refraction out of core
  
  • reflects more times, keeping angle larger than critical angle
• lower change of angle
• quicker signal / improved data transfer
• less distortion

Question 12

Explain how the difference inrefractive index of blue and red light results in a change in the pulse of white light by the time it leaves the fibre.

Answer 12

• Blue travels slower than red due to the greater refractive index
• Red reaches end before blue, leading to material pulse broadening

Question 13

Discuss changes that could be made to reduce the pulse broadening

Answer 13

• Use of monochromatic source so speed of pulse constant
• Use of shorter repeaters so that the pulse is reformed before significant pulse broadening has taken place.
• Use of monomode fibre to reduce multipath dispersion